1. Field of the Invention
The present invention relates to a video display appliance such as a television receiver, a computer monitor, etc., and more particularly to a video display appliance including a device for eliminating the electric field emitted from a cathode ray tube.
2. Description of the Prior Art
A common video display appliance comprises a cathode ray tube (CRT), a cabinet and a back cover. The cabinet and the back cover are made of a thermoplastic material and coupled to each other so as to accommodate the CRT therein. The CRT has a high vacuum space therein, and comprises a funnel-shaped glass envelope having a faceplate and a neck, an electron gun mounted in the neck for radiating electron beams, and a deflection yoke mounted on the outside of the neck.
In such a cathode ray tube as described above, the electron beams radiating from the cathode of the electron gun are controlled, accelerated, and focused by conventional means, and then irradiated on a fluorescent screen formed on the backside of the faceplate. During these processes, horizontal flyback pulses generated by a flyback transformer are applied to a deflection yoke coil, and the high voltage from a high voltage generation section is applied to an anode, thereby generating electric fields.
In CRT as used in a display appliance, a measurable electric field may be generated by the flyback voltage pulses from the horizontal deflection circuit. With the growing use of CRT monitors in many business and recreation activities, concern has also grown about the danger of emitted electric fields to the users.
CRT manufacturers are continually attempting to achieve acceptable radiation levels according to known measuring protocols. Several methods of cancelling the electric fields in front of a CRT are known. One example is illustrated in FIG. 1.
With reference to FIG. 1, a CRT comprises a deflection yoke 2 for deflecting the electron beams radiating from an electron gun 1, a fluorescent screen 4 which radiates by the collision of the electron beams through a shadow mask 3, a high voltage generation section 5 for generating a high voltage, an anode 6 for loading the high voltage generated in the high voltage generation section 5 (for example, 24 KV or so), and a shielding panel 7 mounted on the faceplate of the CRT.
In a CRT of the above construction, the electron beams radiating from the electron gun 1 are vertically or horizontally deflected by the deflection yoke 2. The deflected electron beams come into collision with the fluorescent screen 4 through the shadow mask 3. The fluorescent screen 4 radiates by such collision and, as a result, an image is displayed on the CRT.
At this stage, horizontal flyback pulses generated by the flyback transformer (not shown) are applied to the coil of the deflection yoke 2 and high voltages generated in the high voltage generation section 5 are applied to the anode 6, thereby causing generation of the electric fields (E) expressed by ##EQU1## wherein E is a strength of electric field, V is a voltage being applied to the deflection coil, and d is a distance of the coil.
According to the above equation, the electric field (E) is produced in proportion to the current applied thereto. Many countries have set up statutory restrictions on electric waves, and those skilled in the art have suggested approaches to eliminate the electric waves.
One approach suggested is to provide an electric field eliminator for eliminating the electric fields generated from the deflection coil on the upper part of the deflection yoke, and a coil for offsetting the electric fields on the front of the deflection yoke. To eliminate electric fields, metallic shield plates are provided on the image board coupled to the backside of the neck and on the main board positioned in the lower part of the neck. Also, to eliminate any electric field in front of a CRT, a shielding panel 7, as shown in FIG. 1, is provided on the faceplate of the CRT. Such a panel 7 is a conductive, transparent glass panel coated by indium-tin oxide.
A conventional CRT as illustrated in FIG. 1 can shield the electric fields with a panel attached to the faceplate of the CRT. However, such panels require an expensive special treatment. It is thus difficult to produce the panels in large quantities. Also, the transmittivity of the CRT monitor is lowered by such a shielding panel, thereby resulting in deterioration of the brightness of the monitor. In addition, since the shielding panel is made of glass, the operator is likely to feel eye strain due to the reflection of the panel when attaching the panel to the front of the CRT.